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Fabrication and Applications of Fiber Bragg Grating- a Review Adv. Eng. Tec. Appl. 4, No. 2, 15-25 (2015) 15 Advanced Engineering Technology and Application An International Journal http://dx.doi.org/10.12785/aeta/040202 Fabrication and Applications of Fiber Bragg Grating- A Review Sanjeev Dewra1, Vikas2 and Amit Grover2,∗ SBSSTC Ferozepur, Punjab, 152004, India Received: 24 Sep. 2014, Revised: 17 Mar. 2015, Accepted: 19 Mar. 2015 Published online: 1 May 2015 Abstract: In this paper, the brief introduction of Fiber Bragg Grating, its significant applications, sensing principles, properties, fabrication and the basic designing of FBG have been discussed. FBG’s are relatively simple to manufacture, small in dimension, low cost and exhibits good immunity from the electromegnatic radiations. The former inceptions and the essential techniques of fiber Bragg grating fabrication are described. This paper presents a comprehensive and systematic overview of FBG technology. Keywords: Fiber Bragg grating, Current applications of Fiber Bragg Grating, Fiber Bragg Grating sensors. 1 Introduction A Fiber Bragg Grating is revealing the core of SMF to a periodic model of passionate ultra violet light. The spotlight generates a stable increase in the refractive index of the core of fiber to produce a fixed index modulation in the direction of the exposure pattern and this fixed index modulation is known as grating. At each periodic refraction small amount of light is reflected. When the grating period is about half the input wavelength of light, all the reflected light signals merge comprehensibly to Fig. 1: Fiber Bragg grating in optical fiber[3] one large reflection at a specific channel. This is known as the Bragg condition. The development of permanent grating in optical fiber was first introduced by Hill et al. in 1978 at Canadian communication research centre, strain by stretching fiber and as a feedback mirror for Ottawa, Canada [1,2]. The FBG is depicted in Fig 1. By laser. The photosensitivity was overcome by Meltz et al. temperature and strain tuning of FBG, Spectral [3] who acknowledged from the work of Garside and measurement is not done directly. A narrowband Bragg Lam [4] that it was a 0two photon process, which could grating filter had been created over entire 1m length of be made much more proficient if it was a one photon fiber, known as Hill grating. process at a wavelength in the 245 nm Germania oxygen-vacancy defect bands [5]. The wavelength of the UV light is 244 nm that corresponds to one half of 488 1.1 Formation of Grating and Fabrication nm, to generate the Hill gratings the wavelength of the Techniques blue argon laser line used [6]. In the core of the fiber, the two overlapping UV light beams interfere generating a For use in fiber optic communication it was recognize that periodic interference pattern that has equivalent periodic grating in an optical waveguide have numerous potential index grating. This technique is known as the transverse applications in the fabrication of devices and it was holographic technique. The two beam interferometer depicted that ”Hill grating” could be used as a sensor for arrangement for side-writing FBG is shown in fig 2. ∗ Corresponding author e-mail: amitgrover [email protected] c 2015 NSP Natural Sciences Publishing Cor. 16 S. Dewra et. al. : Fabrication and Applications of Fiber... Fig. 3: Bragg grating fabrication apparatus based on a zero order null diffraction phase mask. The duty cycle of the phase mask is chosen to be 50% [7] Fig. 4: Fiber Bragg for reflection of the incident mode at the λ [9] Fig. 2: Two-beam interferometer arrangement for Side-writing FBG [5] methods have been superseded by the phase mask technique is prepared from flat slab of silica glass that is transparent to UV light [10]. The optical fiber is located in contact with corrugation of the phase as depicted in the The holographic technique for grating fabrication has figure no. 3 UV light that is incident normal to the phase two principle advantages. Bragg grating could be photo mask passes via a periodic corrugation of the phase mask imprinted in the core of fiber without removing the glass [7]. Generally, most of the deflected light is enclosed in cladding. Moreover, the period of the photo induced the 0, +1 and -1 diffracted orders. Though, the phase grating depends upon the angle between the two mask is design by controlling the depth of the corrugation interfacing coherent ultraviolet light beams. Thus even in the phase mask, to suppress the diffraction into the zero through UV light is used to fabricate the grating, Bragg order. Experimentally in the zero-order the amount of grating could be made to function at much longer light can be decreased to less than 5% with approximately wavelengths in a spectral region of interest for devices 40% of the total light intensity divided equally in the 1 that have application in fiber optic communication and orders. The two 1 diffracted order beams interfere to optical sensors [8]. produce a periodic pattern that photo imprints an Previously, FBG was first fabricating using the equivalent grating in the optical fiber [8]. This period is internal writing [1] and holographic technique [3]. These independent of the wavelength of ultra violet light c 2015 NSP Natural Sciences Publishing Cor. Adv. Eng. Tec. Appl. 4, No. 2, 15-25 (2015) / www.naturalspublishing.com/Journals.asp 17 irradiating the phase mask corrugation depth required to attain light is a function of the wavelength and the optical dispersion. The phase mask method has the improvement of simplifying the manufacturing procedure for Bragg grating with a high performance. In contrast with the holographic system, the phase mask method provides easier alignment of the fiber for lower coherence requirements. The Bragg wavelength of the fiber will shift by 2nm. The phase mask technique is very flexible in that it can be used to fabricate gratings with controlled spectral response characteristics [11]. For example, the spectral response of finite length grating with a uniform index modulation along the fiber length has secondary maxima on both sides of the main reflection peak [12]. If the shape of the index modulation along the fiber length is given a bell-like functional shape, these secondary Fig. 5: FBG in OFC system maxima can be suppressed this process is known as apodization [13]. Apodized fiber gratings have been fabricated using the phase masks techniques and suppressions of the side lobes of 30-40 db have been The recent applications of the FBG are: achieved [14,15]. The phase mask techniques have also –Fiber Bragg Grating for Dense WDM been extended to the fabrication of apodized fiber grating. –Fiber Bragg Grating for optical add drop multiplexer Aperiodic or chirped grating are desirable for making –Fiber Bragg Grating as erbium-doped fiber amplifier dispersion compensators [16]. Another approach to Pump laser stabilizer grating fabrication is the point-by-point technique [17]. In –Fiber Bragg Grating as Optical amplifier gain this method each index perturbation of the grating are flattening filter written point-by-point. However, it has been used to fabricate micro-Bragg grating in optical fiber [18] but is most valuable for making coarse gratings with pitches of 2.1 Fiber Bragg Grating for dense wavelength the order of 100m that are required LP modes converters and polarization mode converters [19]. Because of their division multiplexing use in long period fiber grating band-rejection filters and fiber amplifier gain equalizer the interest in coarse period To attain greater spectral efficiency value and total grating has increased [20]. information capacity and to reduce the performance degradation caused by transmission impairments, the system investigation is of great importance. Therefore it is 1.2 Fundamental Properties of Gratings essential to calculate dense WDM transmission for broadband access. Furthermore it is important to evaluate In the core of fiber, the index perturbation is a periodic Dense WDM direct system parameters with currently structure identical to a volume hologram or a crystal available and reasonable priced optical components [23]. lattice that act as a stop-band filter [8]. A narrow band of The fiber Bragg grating technology performs vital role in the incident optical field within the filter is reflected by DWDM system. With the help of the fiber Bragg grating successive, coherent scattering from the index variations we can filter out the particular wavelengths from the [21,22]. Each reflection from a crest in the index system as shown in the Fig. 6 perturbation is in phase with the next one at ?B, as depicted in Fig.4 and any change in fiber properties like temperature, strain that varies the modal index or grating 2.2 FBG for OADM pitch will change the Bragg wavelength [9]. The grating filter characteristics can be understood and modeled by An Optical add drop multiplexer is a device that used in numerous approaches . WDM systems for multiplexing and routing different channels of light into or out of a SMF. This is a type of optical node, that is normally used for the production of 2 Current Applications of FBG optical telecommunications networks. Add/remove here refer to the ability of the device to add one or more new Fiber Bragg grating performs very important role in the channels to an existing multi-wavelength WDM signal optical fiber communication system. The possible use of and to remove one or more channels, passing those FBG in communication system as shown in Fig. 5 signals to another path of network [24]. A traditional c 2015 NSP Natural Sciences Publishing Cor. 18 S. Dewra et. al. : Fabrication and Applications of Fiber... Fig. 6: FBG in DWDM system [23] Fig. 7: OADM with the help of FBG[25] Optical Add/Drop Multiplexer comprises three stages such as an optical demultiplexer, an optical multiplexer manufacturing, this approach has proven to be practical and between them a method of reconfiguring the routes and cost-effective [26].
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